It is proposed to map the genome of tobacco rattle virus. This split-genome virus is composed of two rod-shaped ribonucleoprotein particles each containing a genomic RNA molecule. A determination will be made of the number of proteins coded for by each of the viral RNAs and the arrangement of the responsible cistrons on the virion RNAs. An analysis will be made of the number of non-virion single-stranded RNA species generated during infection and these will be characterized as to the cistron(s) each contains and the virion RNA from which each is generated. An attempt will be made to deduce the mechanism by which these monocistronic messengers are generated from the multicistronic virion RNAs. The size of the smaller of the two virion RNA species varies considerably between strains and an effort will be made to determine the molecular nature of the differences. Finally, the nucleotide sequence will be determined of one of the smallest of the smaller virion RNA species, that of the CAM strain which has a size of 0.7 times 10 to the 6th power daltons.